Generally, “zeolite” is a broad term for crystalline porous aluminosilicates having SiO4 and AlO4 a tetrahedra as the basic units of the structure. The International Zeolite Association (IZA) has registered 179 different zeolites structures. Of these 179, only 18 are reported to be used in commercial operation. Zeolites have so far been extensively used in the petroleum industry and could still play an important role in refining, fine chemicals and petro chemistry and are described well by W. Vermeiren and J.-P. Gilson in the paper published in Topics in Catalysis 49 (2009) 1131. The industrial application value of zeolite is generally determined by the uniqueness of structure of the particular zeolite in question and the production cost of the said zeolite. Some zeolite structures, for example, MFI (referred as Mobil Five), FAU (referred as Faujasite), MOR (referred as Mordenite) are very versatile materials, i.e. their properties can be tuned to the specific requirements of different applications.
Among zeolite frameworks discovered in recent years, MWW structure is particularly useful in industry and is attracting a lot of attention. Zeolites with MWW structure are known to firstly crystallize as layered precursors intercalated with organic template molecules. The dehydroxylation and condensation between the two dimensional lamellar MWW precursors upon calcination in air lead to the formation of 3D MWW zeolite. Zeolites having MWW structure type, such as MCM-22 possess two independent pore systems. One system consists of two-dimensional sinusoidal 10-member ring (10-MR) channels with an elliptical ring cross section of 4.1 Å×5.1 Å. The other is composed of large 12-MR super-cage connected by 10-MR windows. These details are documented by M. E. Leonowicz et. al., in the paper published in Science 264 (1994) 1910.
Due to this unique structure of combination of both 10-MR and 12-MR channel systems, MCM-22 zeolite has been studied extensively as shape selective catalyst for many hydrocarbon conversions as well as an useful adsorbent for many separation and purification processes which are useful in the petroleum, petrochemical and refining industries. For example, U.S. Pat. No. 5,107,047 (1992) describes the application of MCM-22 zeolites for isomerization of olefins. Similarly, U.S. Pat. No. 4,992,615 (1991) describes alkylation of iso-paraffin with light olefins. In addition, application of MCM-22 for the production of cumene and ethyl benzene in liquid phase by alkylation of benzene with propylene or ethylene has been documented in the Encyclopedia of Chemical Processing, (ed. Lee S), Taylor & Francis, New York p. 603 and p 927. In a recent patent publication WO 2007058705 (2007), it is shown that MCM-22 is an efficient catalyst for the alkylation of benzene with long chain olefins. A process for purification through removal of olefins from the BTX stream has been described in U.S. Pat. No. 6,500,996 (2002).
MCM-22 type material was first reported by Rubin and Chu in U.S. Pat. No. 4,954,325 (1990), using hexamethyleneimine as a structure directing agent, under hydrothermal conditions in the range of 80-225° C. for 24 hours to 60 days. The said patent discloses that the source of silica must be comprised predominantly of solid silica, for example at least about 30 wt % of solid SiO2 in order to obtain the crystalline MCM-22 product. The use of a silica source containing at least about 30 wt % solid silica, e.g., Ultrasil, (a precipitated, spray dried silica containing about 90% silica,), or HiSil (a precipitated hydrated SiO2 containing about 87 wt % silica, 6 wt % free H2O and about 4.5 wt % bound H2O of hydration and having particle size of 0.02 micron) favour the formation of crystals of MCM-22. The patent further discloses that the use of sodium silicate (comprised of about 28.8 wt % SiO2, 8.9 wt % of Na2O and 62.3 wt % H2O) yields very little or none of the crystalline MCM-22 (claim 24, and Column 5, lines 32-50, U.S. Pat. No. 4,954,325). Therefore, it limits the range of silica sources which can be used for MCM-22 synthesis having high crystallinity.
Similar teaching are also available in the disclosures of U.S. Pat. No. 5,284,643 (1994) and in U.S. Pat. No. 5,382,742 (1995) wherein gallium containing zeolite MCM-22 has been described.
U.S. Pat. No. 7,326,401 (2008) describes a process for the production of MWW type zeolites having elements with large ionic radius (e.g., Ti, V, Sn etc) in the frame work. A four step method for their manufacturing has been reported, as (i) a step of heating a mixture containing template compound, a compound containing Group 13 element, of the periodic table, a silicon containing compound and water to obtain precursor (A); (ii) a step of acid treating the precursor (A) obtained in the first step; (iii) a step of heating the acid treated precursor (A) obtained in the second step together with a mixture containing a template compound and water to obtain a precursor (B); and (iv) a step of calcining the precursor (B) obtained in the third step to obtain a zeolite substance. Clearly, the procedure disclosed in U.S. Pat. No. 7,326,401 is complicated with too many steps for application purposes in a scaled up manner.
Usually zeolite synthesis is a long duration process and in many cases it takes about a week to obtain the zeolite with a well defined crystalline structure. In order to improve the crystallization of zeolites in general and to reduce the required crystallization period in particular, seeding of the synthesis mixture is a well known technique. Addition of seed enhance the rate of nucleation or crystallization of the zeolite material. Usually it is the same type of zeolite crystal added to the synthesis gel for enhancing the rate of nucleation or crystallization.
U.S. Pat. No. 4,954,325 (1990) and 5284643 (1994) indicate that the crystallization of MCM-22 zeolite is facilitated by the presence of 0.01-1.0 percent (based on total weight) of crystalline product.
U.S. Pat. No. 4,560,542 (1985) discloses a method for the preparation of zeolites (zeolite Beta and zeolite ZSM-8), using a low water and low alkali metal containing gel. An organic templating agent was added to a preformed metallosilicate gel containing less than a specified amount of water and low alkali metal content and maintaining the mixture at crystallization conditions until the crystallization is complete. The method for the preparation of the zeolite, as specified in U.S. Pat. No. 4,560,542 comprises (i) forming a metallosilicate gel having less than 10 moles H2O per gram atom of silicon and an alkali metal content of less than about 0.4 atom per atom of silicon, said metallosilicate gel having been prepared by reacting a source of silica, a source of alumina and water to form a metallosilicate hydrogel, washing the said hydrogel to remove at least a portion residual soluble salts, and drying the washed hydrogel to form a gel having less than 10 moles H2O per gram atom of silicon and (ii) subsequently, mixing the said organic templating agent with the said reaction mixture having a mole ratio of H2O/SiO2 ranging from 2-10.
U.S. Pat. No. 5,330,736 (1994) discloses preparation of zeolite L from an aqueous mixture that contains from 0.1 to about 10% by weight of an amorphous metallosilicate gel which does not contain zeolite L. Using this mixture zeolite L can be made in a much shorter time than that with a synthesis mixture without the seeding gel and without any substantial impurity of other zeolites.
U.S. Pat. No. 6,667,023 (2003) discloses a process for synthesizing MFI type zeolite in the absence of an organic template but in the presence of an amorphous metallosilicate nucleating gel with a SiO2/Al2O3 ratio from about 12 to about 17, as seed material. The SiO2/Al2O3 ratio of the said amorphous metallosilicate nucleating gel, employed as seed, is very critical in realizing a well crystalline material and a deviation on either at lower end or at higher end from the specified SiO2/Al2O3 ratio of the amorphous seed material results in drastic reduction of the crystallinity of the resultant MFI zeolite, e.g. to less than 70% or less than 50% or even less than 20%. In general, more the said deviation from SAR, more is the decrease in crystallinity of the resultant zeolite. In addition, the preparation of the said seed itself requires aging of several weeks together.
U.S. Pat. No. 5,558,851 discloses a method for crystallization of zeolite within the shaped particles while eliminating an external liquid crystallization phase which must be treated or disposed of after the crystallization is complete. In the specification section it is described that the addition of seed crystals is not a requirement of this process, however the process involves the use of a templating agent.
CN1951811A discloses octagonal zeolite synthesizing method which involves synthesizing under alkaline conditions, hydrothermal crystallization and adopting a low-molecular polyalcohol compound as molding agent without zeolite seed.
WO2007075383A2 discloses a process for preparing zeolite involving formation of a reactant gel that forms a precursor crystalline phase; crystallizing the gel; adding recrystallization agent; and completing crystallization. In WO2007075383, X-ray crystalline precursor phase is converted to other crystalline phase using a templating agent.
Corbin et al., in the U.S. Pat. No. 7,014,837 (2006) discloses the preparation of Zeolite A using pre-formed amorphous precursor with small addition of Zeolite A as seed in presence of tetramethyl ammonium hydroxide.
The effects of seeding in the synthesis of zeolite MCM-22 in the presence of hexamethyleneimine was reported by Isao Mochida et. al in the paper published in zeolites Zeolites 18 (1997) 142. The seeds were prepared from an aluminosilicate gel containing hexamethyleneimine and had a very low crystallinity. Addition of the hexamethyleneimine containing MCM-22 seed resulted in a reduction of the crystallization period of MCM-22 from a synthesis mixture that already contained the templating agent (hexamethyleneimine).
Thus, it is clear from the discussion on the prior art, that there are certain limitations in the art of preparation of MCM-22 zeolite in an easy and convenient manner in a short period, without any additional steps or need for modification of the existing commercial manufacturing facilities for production of zeolite.
Thus the present invention seeks to overcome the difficulties, disadvantages and deficiencies faced by the prior art by providing a method for preparing the MWW type of zeolite in an easy, convenient and rapid manner.